Flame retardant polyamides

ABSTRACT

Flame retardant polyamide compositions are provided which contain melamine as the flame retardant. Flame retardant benefits are particularly realized by a composition which is a blend of melamine with a copolymer of polyamide-6.6 and 6. Use of heat stabilizers in the preferred compositions minimizes the thermal aging properties of the copolymer compared to polyamide-6.6.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.244,371, filed as PCT/US92/10558 Dec. 14, 1992, now U.S. Pat. No.5,476,887, and entitled Flame Retardant Polyamides.

BACKGROUND OF THE INVENTION

This invention relates to certain flame retardant polyamides and moreparticularly to such polyamides where the polyamides are copolymers ofpolyamide 6.6.

The use of melamine and melamine derivatives such as melamine cyanurateas flame retardant agents for polyamides has been known for many years(U.S. Pat. No. 3,660,344 and U.S. Pat. No. Re 30,402). In the case ofpolyamide 6.6, which is specifically used for its better thermal agingproperties, versus polyamide 6, this has not been achieved with melaminebecause its volatility creates a large amount of melamine white molddeposition. Additionally, blooming of melamine from molded pieces undervarious simulated use conditions has been noted previously (U.S. Pat.No. 4,525,505) as a reason why this art has not been practicedcommercially. The volatility of melamine and its tendency to sublime attypical polyamide 6.6 molding temperatures (270° C.-290° C.) hasgenerally rendered melamine-containing polyamide 6.6 compositionsunattractive from a commercial point of view. The alternative tomelamine has been the use of melamine derivatives or its condensationproducts such as melamine cyanurate; but, although these have presentedsolutions to the melamine blooming issues, they have proved difficult toscale-up to typical commercial processes without some loss offlammability control.

The object of the invention is to provide unreinforced flame retardantpolyamide molding compositions which have melamine as the flameretardant and which consistently meet the UL94 V-O criteria at all thespecified thicknesses according to standards described for commercialacceptability based on statistical representation whilst at the sametime minimizing any blooming of melamine during the molding processwithout reducing the thermal resistance properties of the polyamides.

SUMMARY OF THE INVENTION

The present invention provides a flame retardant polyamide compositionconsisting essentially of:

(a) a polyamide having a melting point of below about 250° C.; and

(b) 10-20% by weight of melamine based on the weight of the composition.

Also provided is a filled polyamide composition wherein 20-50% by weightof a filler is added to the aforesaid composition. Further provided is ashaped article formed from the aforesaid unfilled or filled polyamidecompositions.

DETAILED DESCRIPTION OF THE INVENTION

The object of the present invention has been accomplished by combining apolyamide having a melting point of below about 250° C. with melaminewhereby the resulting composition contains 10-20% by weight of melamine.

Specifically, the melting point of polyamide 6.6 (m.p.=about 260° C.) islowered to a temperature below about 250° C., preferably below about245° C., and most preferably in the range of about 230° C.-245° C. Thisis accomplished by copolymerizing polyamide 6.6 with another monomercopolymerizable with it and which provides a copolymer melting point inthe ranges stated above. Additional benefits in terms of long term agingare realized by the addition of a small amount of a heat stabilizer,preferably copper iodide. Other additives well known to those in thepolyamide processing and molding art can be used as desired.

Suitable polyamides include those described in U.S. Pat. Nos. 2,071,250,2,071,251, 2,130,523, 2,130,948, 2,241,322, 2,312,966, 2,512,606, and3,393,210. These polyamides are well known in the art and embrace thosesemi-crystalline and amorphous polymers having a molecular weight of atleast 5,000 and commonly referred to as nylons.

The polyamide can be produced by condensation of equimolar amounts of asaturated dicarboxylic acid containing from 4-12 condensation ofequimolar amounts of a saturated dicarboxylic acid containing from 4 to12 carbon atoms with a diamine, in which the diamine contains from 4 to14 carbon atoms. Excess diamine can be employed to provide an excess ofamine end groups over carboxy end groups in the polyamide.

Examples of polyamides include copolymers of polyhexamethylene adipamide(66 nylon), which melt below 250° C., polyhexamethylene azelaamide (69nylon), polyhexamethylene sebacamide (610 nylon), and polyhexamethylenedodecanoamide (612 nylon), the polyamide produced by ring opening oflactams, i.e., polycaprolactam, polylauric lactam,poly-11-amino-undecanoic acid, bis (paraaminocyclohexyl) methanedodecanoamide. It is also possible to use in this invention polyamidesprepared by the copolymerization of two of the above polymers orterpolymerization of the above polymers or their components, e.g., forexample, an adipic, isophthalic acid hexamethylene diamine copolymer.Preferably, the polyamides are linear with a melting point in excess of200° C., but below 250° C. Specifically, modified PA6.6 (particularly a6.6-6 copolymer), PA6, PA6.10, PA6.12, PA11, PA12, PA12.12, PA6/6.6,etc. may be used. Particularly preferred because of high physicalproperties is a 6.6 polymer whose melting point is lowered bycopolymerization with another monomer such as caprolactam, adicarboxylic acid of 7-14 carbon atoms, m-benzenedicarboxylic acid,o-benzenedicarboxylic acid, or p-benzenedicarboxylic acid. The importantparameter being that the polyamide used has a melting point below about250° C.

There are no special restrictions as to molecular weight of thepolyamides. Preferable are polyamides with a relative viscosity (RV)according to ASTM D789 of 20 to 70, preferably 40-60.

The compositions of the invention are prepared by thoroughly mixingmelamine, the polyamide copolymer, and any of the other fillers andadditives used in an extruder/screw combination which is specificallyintended for the compounding of powders into thermoplastic resin. Duringmixing, particularly in an extruder, the hold-up time should be shortenough so as to avoid or minimize a lowering of the polyamide copolymermelting point and consequent crafting of the melamine to the copolymer.While hold-up time will vary somewhat depending upon the exact copolymercomposition and temperatures used, a hold up time less than 30 secondsis sufficient. The resulting extrudate is cooled, pelletized and driedbefore molding the resulting composition into shaped articles.

The flame retardant polyamide compositions can also contain conventionaladditives, fillers, and reinforcing agents such as lubricants,mould-release agents, stabilizers, dyes, pigments, color concentrates,flow agents, glass and organic fibers, chalk, quartz, and mineralfillers such as magnesium hydroxide. Fillers and reinforcing agents areused at concentrations of 20-50% based on the weight of the composition.In addition, nucleation agents for the polyamide copolymer can be addedso as to increase the crystallinity of the copolymer. Increasedcrystallinity results in an increase in copolymer physical propertiesover those of the unnucleated copolymer.

It has been found previously that polyamides containing melamine do notpass the UL94 V-O classification at all thicknesses of test specimens,particularly when production is scaled up from small laboratory tolarger commercial scale equipment (due in part to difficulties incontrolling temperature and dispersion of the flame retardant in theresin); they fail by having flaming drips which ignite cotton waddingplaced directly under the test specimen during the test, particularlythose 1.6 mm in thickness. As a result, these compositions receive a V-2classification. Surprisingly, the polyamides of the present inventionpass the V-O classification with statistically significant regularity;thus with commercial reliability. Additionally, no evidence ofpreviously reported blooming is observed on molded parts upon aging atelevated temperatures and humidity.

In the examples which follow, 100 test bars (50 at each conditiondescribed in the UL94 test) were tested in order to give a percentage ofthe total which fail by igniting the cotton. This percent failure canthen through experience of the manufacturing process and the variabilityin the test method itself be correlated with the ability to reliablymake commercial flame retardant resins which consistently pass the UL94test at V-O. In these examples, parts and percentages are by weightunless indicated otherwise.

EXAMPLES EXAMPLES 1-3

A polyamide 6.6-6 copolymer was prepared by polymerizing hexamethylenediamine, adipic acid, and caprolactam to give a copolymer containing 85%6.6 and 15% 6, and having a m.p. of about 235° C. and a relativeviscosity of about 45-55 according to ASTM D789. Various amounts ofmelamine and melamine cyanurate (as a control) were mixed with thecopolymer using a corotating twin screw extruder having a dietemperature of 250° C. and having a flat temperature profile of about245° C. over the length of the extruder up to the die. About 1% of otheradditives such as waxes and heat stabilizers (including 0.05% copperiodide) were also mixed into each composition. As a control, acommercially available flame retardant (FR) nylon was also tested; thisresin contained 72% polyamide 6.6, 17% polyamide 6, 10% melaminecyanurate, and 1% other minor additives. Extrusion of this control wasat about 270° C.

Test bars were molded at 240° C. to give bars 127 mm×12.7 mm×1.6 mm (or0.8 mm). Physical properties were tested according to ASTM ZD638, D790,D256, D789 and UL94. The results are shown in Table I.

                                      TABLE I                                     __________________________________________________________________________                 FR  Example Number                                                            Nylon                                                                             A   B   1   2  3   C                                         __________________________________________________________________________    Melamine cyan. (%)                                                                         10  --  --  --  -- --  12                                        Melamine (%) --  6   8   10  12 15  --                                        PROPERTIES                                                                    T.S. at Yield (MPa)                                                                        82.1                                                                              83.4                                                                              81.5                                                                              77.1                                                                              76 --  82.4                                      T.S. at Break (Mpa)                                                                        76.2                                                                              76.5                                                                              70.1                                                                              62.4                                                                              68.3                                                                             --  77                                        Elongation at Yield (%)                                                                    3.8 3.9 4.2 4.2 4.1                                                                              --  3.6                                       Elongation at Break (%)                                                                    9.3 7.9 10.7                                                                              17.1                                                                              9.9                                                                              --  5.5                                       Flex Mod (GPa)                                                                             3.48                                                                              3.09                                                                              3.19                                                                              3.27                                                                              3.42                                                                             --  3.42                                      NI (J/M)     33  32  33  31  31 --  32                                        UNI (J/M)    834 51  1061                                                                              940 779                                                                              --  676                                       STD UL94 at 1.6 MM                                                                         V-O V-2 V-2 V-2 V-O                                                                              V-O V-O                                       STD UL94 at 0.8 MM                                                                         V-O V-2 V-O V-O V-O                                                                              V-O V-O                                       UL94% fails at 1.6 MM                                                                      25  50  20  15  1  0   25                                        UL94% fails at 0.8 MM                                                                      --  --  --  --  0  0   --                                        RELATIVE     48.2                                                                              45.8                                                                              41.7                                                                              40.1                                                                              39.2                                                                             --  42.2                                      VISCOSITY                                                                     __________________________________________________________________________

Test bars were aged for 8 weeks in an oven maintained at 120° C. and50-100% relative humidity. No evidence of surface blooming of melamineoccurred during this period.

EXAMPLE 4

A filled polyamide copolymer composition was prepared using thecopolymer of Examples 1-3. The procedures as set forth in these Exampleswere repeated on a composition which contained 20% melamine, 45%magnesium hydroxide as a mineral filler, 3% of a black colorconcentrate, and 0.5% of aluminum distearate as a lubricant.

Test bars were molded as described in Examples 1-3 and passed the UL94test at 1.6 mm as a V-O classification.

I claim:
 1. A flame retardant polyamide composition consistingessentially of:(a) a copolymer of polyamide 6.6 and at least one othermonomer selected from the group consisting of a dicarboxylic acid of7-14 carbon atoms, m-benzenedicarboxylic acid, o-benzenedicarboxylicacid, and p-benzenedicarboxylic acid; said copolymer having a meltingpoint below 250° C.; and (b) as the only flame retardant, 10-20% byweight of melamine, based on the weight of components (a) plus (b) only;and, optionally, (c) at least one additive selected from the groupconsisting of glass fibers, organic fibers, mineral fillers, lubricants,mold-release agents, stabilizers, dyes, pigments, color concentrates,flow agents, chalk, and quartz.
 2. A composition of claim 1 which isfree of fillers and reinforcing agents.
 3. A composition of claim 1which comprises at least one additive selected from the class consistingof mineral fillers and reinforcing agents.
 4. A composition of claim 3wherein the amount of the additive is about 20-50% by weight, based onthe weight of copolymer of polyamide 6.6 plus melamine.
 5. A compositionof claim 3 which is free of mineral fillers.
 6. A composiiton of claim4, wherein the reinforcing agent is glass fibers.
 7. A compositionaccording to claim 1 wherein the melamine concentration is in the rangeof 12-20% by weight.
 8. A composition according to claim 1 wherein aheat stabilizer is contained in the composition.
 9. A compositionaccording to claim 8 wherein the heat stabilizer is copper iodide.
 10. Acomposition of claim 1 in the form of a shaped article.
 11. Acomposition of claim 2 in the form of a shaped article.
 12. Acomposition of claim 3 in the form of a shaped article.